Generally, EVOH is very excellent in such properties as transparency, gas barrier property, aroma-retaining property, solvent resistance and oil resistance. Owing to these characteristics, EVOH is molded into films, sheets, or containers such as bottles, for use as packaging materials for foods, drugs, industrial chemicals, agrochemicals and so forth.
In molding EVOH, melt molding is generally performed. By said melt molding, EVOH is processed into various forms for practical use, such as films, sheets, bottles, cups, tubes and pipes. The processability (moldability) of EVOH is therefore very important. In many instances, EVOH is coextruded with a substrate material such as a polyolefin resin via an adhesive layer into laminates so that EVOH can be provided with mechanical strength, moisture resistance and heat-sealability, among others. In those cases, the interlaminar adhesion of the laminates is also important.
Thus, for producing practical products utilizing the above-mentioned characteristic features of EVOH, EVOH is required to be fully satisfactory from the viewpoints of the possibility of long-run melt molding operations, the appearance of moldings (absence of fish eyes or striate) and the interlaminar adhesion. Since, however, EVOH is inferior in melt moldability to general purpose resins for molding, it is necessary to take measures to improve the moldability.
The following publications describe the treatment of EVOH with a boron compound or the incorporation of a boron compound in EVOH.
(1) Japanese Patent Publication S49-20615 discloses a process for producing EVOH having excellent heat stability and mechanical properties which comprises treating EVOH with a boron compound.
(2) Japanese Kokai Tokkyo Koho S59-192564 discloses a gas barrier laminate comprising a layer of EVOH treated with a boron compound as overlaid, on one side or both sides thereof, with a polyolefin layer or layers via an adhesive layer composed of a carboxyl- or hydroxyl-containing modified polyolefin. This laminate shows improved interlaminar adhesion. The boron content of the boron compound-treated EVOH is 0.002 to 0.2% by weight, preferably 0.005 to 0.1% by weight.
(3) Japanese Kokai Tokkyo Koho S55-12108, the applicant of which is the present applicant, discloses the melt molding of a composition comprising a melt-moldable vinyl alcohol copolymer (inclusive of EVOH) supplemented with 0.02 to 2% by weight of a rapid reaction type melt tension improving agent (inclusive of boric acid and boron).
(4) Japanese Kokai Tokkyo Koho S57-34148, the applicant of which is also the present applicant, discloses a resin composition limited in draw down and excellent in shock resistance which comprises 100 parts by weight of EVOH, 2 to 15 parts by weight of an alcohol-based plasticizer and 0.05 to 3 parts by weight of boric acid or borax.
The following publications describe the treatment of EVOH with a phosphoric acid compound or the incorporation of a phosphoric acid compound in EVOH.
(1) Japanese Kokai Tokkyo Koho S52-954 discloses a composition comprising a polyvinyl alcohol polymer (inclusive of EVOH) with 0.001 to 1% by weight of a partial alkali metal salt of a polybasic acid having a specific pKa value (e.g. potassium dihydrogen citrate, potassium dihydrogen phosphate, sodium dihydrogen phosphate, sodium hydrogen tartrate) as added thereto. The purpose of the incorporation of a partial alkali metal salt of a polybasic acid is to improve the heat stability and the processability in thermal molding.
(2) Japanese Kokai Tokkyo Koho S64-66262 discloses a composition comprising EVOH, 0.0005 to 0.05% by weight (calculated as metal) of a salt of a metal of group II of the periodic table (e.g. calcium acetate, magnesium acetate), 0.0005 to 0.05% by weight of an acidic substance having a pKa of not less than 3.5 and a boiling point at ordinary pressure of not lower than 180.degree. C. (e.g. adipic acid, benzoic acid, potassium dihydrogen phosphate, sodium dihydrogen phosphate, aspartic acid) and 0.01 to 0.2% by weight of an acidic substance having a pKa of not less than 3.5 and a boiling point at ordinary pressure of not higher than 120.degree. C. (e.g. acetic acid, formic acid).
(3) Japanese Kokai Tokkyo Koho H02-235952 discloses an EVOH composition comprising EVOH and specified small proportions of an alkali metal acetate, acetic acid, and phosphoric acid or an alkali metal hydrogen phosphate as well as a multilayer container with said composition as the innermost layer. The purpose of incorporating these compounds is to reduce the emanation of odorous components and improve the heat stability.
(4) Japanese Kokai Tokkyo Koho S62-143954, the applicant of which is the present applicant, discloses a method of treating EVOH which comprises treating EVOH with an acid having a pKa of not more than 5.0 (e.g. acetic acid, phosphoric acid, sodium dihydrogen phosphate) and a calcium or magnesium salt of a polybasic acid having an acid group showing a pKa of not more than 3.4 (e.g. calcium dihydrogen phosphate, magnesium dihydrogen phosphate). The purpose of this treatment is to improve the heat stability during melt molding and obtain beautiful moldings free of fish eyes or the like on the occasion of molding into films, for instance.
As mentioned above, several proposals have been put forth concerning the treatment of EVOH with a boron compound or the incorporation of a boron compound in EVOH. Concerning the treatment of EVOH with a phosphoric acid compound or the incorporation of a phosphoric acid compound in EVOH, too, several proposals have been announced.
However, the requirements imposed on the performance characteristics of EVOH in relation to the moldability and quality thereof have recently become much more severe than before and the methods disclosed in the above-cited publications can no longer satisfactorily meet such severe requirements. For instance, according to the methods described in the above-cited publications, improvements are indeed observable with respect to fish eyes or gels with a diameter of not less than 0.1 mm but, as to fish eyes or gels with a diameter of less than 0.1 mm, it is not yet achievable to reduce these to such an extent that they will not cause any trouble from the practical use viewpoint.
Under the circumstances, the present invention has for its object to provide an EVOH resin composition much more improved in melt moldability (long-run moldability), moldings' appearance (reduction in fish eyes and striate), and laminates' interlaminar adhesion as compared with the conventional compositions, and a process for producing the same, and to provide a laminate comprising a layer of said resin composition.